The present invention relates generally to retrievable packers for use in subterranean wellbores and, in a preferred embodiment thereof, more particularly provides a retrievable packer which may be torqued through, and which has high strength and a reduced cross-sectional area.
Retrievable packers are typically conveyed into a subterranean wellbore suspended from a service tool which is, in turn, suspended from tubing extending to the earth's surface. Such packers are generally utilized for two main purposes--to provide a releasable anchor for preventing longitudinal movement relative to the wellbore, and to provide a releasable annular seal between portions of the wellbore above and below the packer.
When the wellbore is lined with tubular protective casing, the anchor function of a packer is usually performed by hardened jaw-like gripping members known as "slips" which, contrary to their name, act to prevent slippage of the packer within the casing. The slips are typically designed to extend radially outward from the packer and, when extended, bite into the casing's inner surface. In this manner, the packer's slips permit forces to be applied to the packer without resulting in movement of the packer within the wellbore. A type of slip known as a "bidirectional" slip permits both tensile and compressive forces to be applied to the packer without producing longitudinal movement of the packer relative to the casing.
The sealing function of the packer is typically performed by multiple ring-shaped "rubbers" located in axially compressible annular recesses formed on the packer's exterior surface. When compressed, the recesses force the rubbers radially outward to seal against the inner surface of the casing. Generally, the axial compression of the recess coincides with the radially outward extension of the slips to thereby "set" the packer in the casing.
It will be appreciated from the foregoing that complex mechanical and/or hydraulic mechanisms are required to efficiently achieve the main functions of the packer. The mechanisms necessarily are located in cross-sectional areas between the inner and outer surfaces of the packer. In recent years it has become increasingly important to compress those mechanisms into smaller and smaller cross-sectional areas of packers to provide for increased flow area through the packers. For example, in some fracturing, acidizing, and gravel packing operations, and combinations thereof, it is highly desirable to utilize a packer having a large flow area therethrough, while simultaneously having the ability to seal against high pressures and resist movement due to high loads applied to the packer.
In past packer designs, as with most engineering designs, trade-offs were typically made in an effort to optimize the designs for intended uses of the packers. In general, a packer which had increased anchoring or pressure sealing capabilities would consequently have a reduced flow area. Conversely, a packer which had a large flow area would usually be restricted in its anchoring and sealing capabilities. These past packer designs are, therefore, unsuitable for use in those operations requiring large flow area, high pressure sealing, and anchoring against high loads.
Another desirable characteristic of retrievable packers is the ability to apply torque through the packer while running in or out of the wellbore. Such torque may be used to enable the packer to pass an obstruction in the casing, operate other equipment, etc. At times, it is also desirable for the packer to be rotated within the casing when torque is applied to the packer and for the packer to rotate as a unit, that is, with all, or substantially all, portions of the packer rotating together.
Mechanisms for permitting torque-through of a packer and for preventing relative rotation of packer portions, as with the anchoring and sealing mechanisms discussed above, are necessarily located in the cross-sectional area of the packer. Therefore, in the past, these features of a packer design typically resulted in decreased flow area, reduced anchoring ability, reduced sealing ability, or a combination thereof.
From the foregoing, it can be seen that it would be quite desirable to provide a packer which efficiently utilizes its cross-sectional area to thereby simultaneously achieve a large flow area, high resistance to axial and radial forces applied thereto, high pressure sealing capability, the ability to torque through the packer, and the ability to prevent relative rotation of portions of the packer. It is accordingly an object of the present invention to provide such a packer.